1. Field of the Invention
The present invention relates generally to composite sandwich panels which are used in aerospace and other applications where light weight and high strength are required. More particularly, the present invention is directed to composite sandwich panels which are designed for use in an environment where flame resistance and/or fire protection is required.
2. Description of Related Art
Composite sandwich panels are widely used in aerospace and other industries where structures are required that are light weight and strong. The sandwich panels typically include a light weight central core structure which is sandwiched between two composite face sheets. The face sheets are adhesively bonded to the core. Various core structures are presently in use with the two main types being rigid foam and honeycomb. Rigid foam cores are advantageous because the face sheets contact the foam over a relatively large surface area which insures a strong bond. In many foam cores, the core will fracture before the adhesive bond between the face sheets and the core fails. Some disadvantages of rigid foam core are that light weight rigid foam is not particularly strong and the fire or flame resistance of many foams is not particularly good.
Honeycomb provides a number of advantages over rigid foam cores. Honeycomb cores, in general, provide much greater strength than rigid foam cores having the same density. In addition, honeycomb can be made from non-flammable materials such as aluminum and other light weight metals. One drawback of a honeycomb core is that the surface area which is available for bonding to the face sheets is much smaller than for foam cores. The honeycomb cells extend transversely between the face sheets so that the only surface available for bonding to the face sheets is the outer edges of the cells. As a result, the overall strength of the majority of honeycomb panels is significantly affected by the strength of the bond between the face sheets and the honeycomb.
Epoxy-based adhesives have been used to bond face sheets to honeycomb cores where the structural strength of the panel is important. For example, honeycomb sandwich panels used for aircraft flooring and other high stress structures have typically utilized epoxy-based adhesives to bond the face sheets to honeycomb core. Epoxy-based adhesives tend to burn relatively easily and produce large amounts of smoke. Attempts have been made to increase the fire resistance of epoxy adhesives by halogenating the adhesives. However, these attempts have not been entirely successful.
Phenolic adhesives are inherently more fire resistant than epoxy adhesives. However, phenolic resins do not have the same strong adhesive properties found in epoxy resins. As a result, phenolic adhesives have been limited to use in honeycomb sandwich panels which are not structural in nature. Such panels include aircraft interior side walls, ceilings and overhead bins.
In accordance with the present invention, it was discovered that structurally strong composite sandwich panels can be made using phenolic adhesives to bond the face sheets to the honeycomb. It was found that certain configurations and combination of phenolic resins and fire protection agents provide low flame, smoke and toxicity (FST) panels which have structural strengths which are equivalent to prior panels using epoxy-based adhesives.
The present invention is based upon a composite sandwich panel which has a core structure that includes a first face and a second face located on opposite sides of the core. At least one face sheet is adhesively bonded to one of the core faces. The face sheet has at least one fiber layer and includes an adhesive layer located adjacent to the face of the core structure. The face sheet further includes a fire protection layer which is located adjacent to the adhesive layer. The fire protection layer is also a phenolic resin. The phenolic resin in the fire protection layer includes a sufficient amount of one or more fire protection agents to provide the face sheet with a flammability level of below 20/20 as measured by the Ohio State University (OSU) heat release test.
Included within the present invention are face sheets which include a single fiber layer. In such single-ply face sheets, the single fiber layer is located between the adhesive layer and the fire protection layer. The phenolic adhesive is present in the adhesive layer in amounts sufficient to provide the necessary peel strength level of at least 80 N/76 mm. The fire protection layer may include a fire barrier layer located adjacent to the single fiber layer wherein the fire protection agent that is located within the fire barrier layer is a thermal barrier material. The fire protection layer also includes an intumescent layer which is located adjacent to the fire barrier layer if present. The intumescent layer includes one or more intumescent agents which are present in a phenolic resin.
The present invention also covers sandwich panels which utilize face sheets having multiple plies. The multiple-ply low FST, high-strength sandwich panels in accordance with the present invention include an inner ply which is made up of a first fiber layer located closest to the core structure and at least one outer ply which includes a second fiber layer wherein the second fiber layer is located within the fire protection layer. As one feature of the present invention, the phenolic adhesive layer in such multiple ply face sheets is located substantially between the interior side of the first fiber layer and the face sheet of the core structure. The fire protection layer begins on the exterior side of the first fiber layer and extends outward through the second ply and any additional plies. The fire protection layer may include only intumescent agents. The fire protection layer may include outer layers which incorporate only intumescent agents with one or more inner layers including thermal barrier material. The peel strength of such multiple ply face sheets is at least 130 N/76 mm.
The present invention is also directed to multiple ply face sheets wherein the phenolic adhesive is located on both sides of the first fiber layer. In this type of face sheet, the fire protection agents are located only in the second ply and/or optionally in additional plies.
The present invention is particularly well suited for use in the aerospace industry where structural panels must meet certain requirements for weight, strength, flammability, smoke and toxicity.
The above discussed and many other features and attendant advantages of the present invention will become better understood by reference to the following detailed description when taken in conjunction with the accompanying drawings.